Flow dynamics during single- and dual-spot laser welding with one common keyhole of 321 stainless steel

The keyhole is unstable with the presence of humps at the keyhole walls during single-spot laser welding (SSLW). The keyhole stability is improved and the presence of humps is reduced during dual-spot laser welding (DSLW). The vortices present at the rear keyhole wall during SSLW disappear during DSLW. There is an upward motion of the melt flow at the rear keyhole wall and the melt flow velocity is higher along the width of the molten pool during DSLW. A sandwich specimen is constructed to investigate the effect of keyhole and melt flow dynamics on the final shape of the weld seam. The results show that DSLW produces smoother, more regular weld beads due to higher stability of the keyhole and molten pool. DSLW produces a weld seam with fewer porosities and a more homogeneous nickel distribution due to the upward motion and absence of vortices in the melt flow. DSLW produces a wider molten pool due to the higher melt flow velocity along the width of the molten pool.